1. Field of the Invention
The invention relates to a process for the thermal treatment of solid materials, in particular refuse, such as domestic and community waste, in which the solid materials are burnt/gasified or pyrolized in a first step with a lack of oxygen, and then, in an afterburning zone, the flue gases from the first step are mixed with an oxygen-containing gaseous medium and are burnt with complete burn-off.
2. Background of the Invention
It is known in the prior art to burn lumpy solid materials, such as for example refuse, in a combustion chamber to which primary air is added, and a downstream afterburning chamber, to which secondary air is added. Usually, in this case, the solid material is moved on a combustion grate. The primary air is fed in beneath the grate and flown through openings in the grate covering into the bed of solid material lying above the grate.
The flue gases which are formed in and above the bed during combustion have a composition and temperature which fluctuate considerably locally and over the course of time. Therefore, in conventional systems, these flue gases are subsequently mixed with the aid of secondary air or secondary air and recirculated flue gas. The secondary air fulfills the following functions:
mixing the gases emerging from the combustion chamber
supplying oxygen in order to ensure burn-off of the gases
cooling of the emerging gases.
The primary air added in the first step is usually sufficient to completely burn the fuel, and the secondary air is used to achieve cross-mixing of the flue gas (mixing of CO-containing gas trains with O2-containing gas trains). To ensure sufficient mixing, the amount of secondary air blown in must be selected to be suitably high. However, this excess air has the drawback of increasing the volume of flue gas.
In order to eliminate this drawback, EP 0,607,210 B1 describes a process for the combustion of solid materials, in which apart from the primary air no further combustion air is fed into the combustion boiler. To improve the poor burn-off of the gases which is caused by insufficient mixing in the afterburning chamber and which leads to high pollutant levels in the flue gas, it is proposed in EP 0,607,210 B1, on the one hand to add sufficient primary air to provide an excess of oxygen as early as in the first step, and on the other hand to inject water steam into the combustion boiler above the combustion space and in the lower area of the afterburning chamber at an ultrasonic speed produced by excess pressure. This process has the drawback that, in the event of there being an excess of air in the first combustion step, much of the nitrogen contained in the fuel is oxidized to form NO, and consequently it is impossible to achieve low NOx emissions.
A further process for the thermal treatment of refuse is known (Beckmann, M. and R. Scholz: xe2x80x9cVergasung von Abfxc3xa4llenxe2x80x9d [Gasification of Refuse] in xe2x80x9cVergasungsverfahren fxc3xcr die Entsorgung von Abfxc3xa4llenxe2x80x9d [Gasification Process for Disposing of Refuse], Springer-VDI-Verlag GmbH, Dxc3xcsseldorf, 1998, pp. 80-109), in which process the volume of primary air beneath the grate is reduced to such an extent that the fuel is gasified and a CO-rich flue gas is formed. In a following, completely separate afterburning chamber, this flue gas is afterburnt with air. Although the considerable reduction in the addition of air in the first step is reported to provide an advantageous clear reduction in the NOx emissions compared to conventional grate combustion systems, hitherto this process has only been carried out on trial scale. The afterburning chamber was completely separate from the combustion chamber and connected by a pipe. The flue-gas stream was homogenized by means of turbulence when it flowed through this pipe. As a result of the small batch size and of the flue-gas stream being guided out of the primary combustion chamber through a connection pipe, it was possible to dispense with a device for mixing the flue-gas stream emanating from the primary combustion chamber without increased concentrations of pollutants being found in the flue gas from the afterburning chamber. However, the use of a pipe to connect the primary combustion chamber with the afterburning chamber represents a drawback in an industrial-scale installation (wear, caking).
The invention seeks to avoid these drawbacks. Accordingly, one object of the invention is to provide a novel process for the thermal treatment of solid materials, in particular refuse, in which the solid materials are burnt/gasified or pyrolized in a first step with a lack of oxygen, and then the emerging gases are mixed with the oxygen-containing medium which is required for complete burn-off and are burnt, in which process local concentration and temperature fluctuations in the flue gas from the first step are eliminated and as a result the pollutant concentrations, in particular the NOx emissions, are minimized.
According to the invention, this is achieved by the fact that, for the purpose of NOx reduction, the flue gases emerging from the first step, before they are mixed with the oxygen-containing medium in a mixing zone, are actively homogenized with the addition of a gaseous, oxygen-free or low-oxygen medium, and the homogenized, low-oxygen flue-gas stream emerging from the mixing zone, before the oxygen-containing medium which is required for complete burn-off is added, passes through a holding zone, the residence time in the holding zone being at least 0.5 second.
The advantages of the invention consist in the fact that the gases emerging from the first step, due to their subsequent homogenization, no longer exhibit any concentration and temperature fluctuations when they are mixed with the burn-off air. The additional residence time for the homogenized gas stream in the holding zone with a lack of air (substoichiometric air ratio) allows the NO which has already been formed to be reduced by the NHx, HCN and CO present to form N2. Consequently, only minimal pollutant emissions are formed in the thermal treatment according to the invention of the solid materials.
It is particularly expedient if recirculated flue gas, water steam, oxygen-depleted air or inert gases, such as for example nitrogen, are used as gaseous oxygen-free or low-oxygen media for homogenization. These gases are advantageously injected into the mixing zone perpendicular to the direction of flow of the flue gases or, in order to improve the homogenization and mixing effect still further, are injected at a certain angle and in the opposite or same direction to the direction of flow of the flue gas from the first step.
Furthermore, it is advantageous if the active homogenization of the flue gases emerging from the first step is carried out with the aid of components (static mixing elements) which are installed in the mixing zone. These installed components divert the flow of the flue gases and consequently cause them to be efficiently and intimately mixed. It is expedient if these installed components have cavities through which a cooling medium, e.g. water, water steam or air, flows.
Finally, it is advantageous for the active homogenization of the flue gases emerging from the first step to be carried out by means of constrictions or widenings of the cross section of the flow channel.
Moreover, it is expedient to control the temperature of the flue gases in the area where the oxygen-containing medium is injected by means of the amount of oxygen-free or low-oxygen gaseous medium which is fed to the mixing zone. This represents a very simple way of keeping the temperature constant.
It is advantageous if a grate system with center-current firing or with countercurrent firing is used as the first step.
Furthermore, it is advantageous if a fluidized bed is used as the first step, since this provides a very good mass and heat transfer effect. Local temperature peaks and locally increased wear to the refractory lining can be prevented. Moreover, the ferrous and nonferrous metals contained in the waste can be recovered from the ash with a very good quality.
It is also expedient if the afterburning zone is a fluidized bed and the oxygen-containing gaseous medium is fed to the entry to the fluidized bed or directly into the fluidized bed. It is then advantageously possible, due to the increased heat transfer caused by the presence of particles, to avoid local hot zones with a high level of thermal NOx formation. Moreover, caking on the heat-exchanger walls is prevented, with the result that the corrosion on the heat-exchanger surfaces is reduced. It is possible to set higher steam pressures and temperatures, allowing a higher thermal efficiency of the combustion installation to be achieved.
Finally, it is expedient if the holding zone is a fluidized bed and the gaseous oxygen-free or low-oxygen medium is fed to the entry to the fluidized bed or directly into the fluidized bed.